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Who cares about
ultrasound safety? or What the ASUM Safety Committee does for
you
Stanley B Barnett,
PhD
INTRODUCTION
Simply, every responsible user of diagnostic ultrasound equipment
should have some understanding of the likelihood of risk to the patient
from the examination. Research has shown that acoustic outputs from
modern ultrasonographic equipment are sufficient to produce biological
effects in some tissue. The likelihood of producing such biological
effects varies according to the examination type and whether, or not,
extraneous factors, such as echo-contrast agents, are introduced.
Whilst it would be unreasonable to expect ultrasound users to become
experts in the subject of radiation safety, reference to a set of
published guidelines together with some background information can
help provide reassuring answers to difficult questions. There is no
substitute for a sensible and sympathetic response to patient enquiry,
particularly if the patient has become disturbed by the occasional
inflammatory article in the popular press. A recent example was the
report that diagnostic ultrasound caused bleeding, the formation of
heat shock proteins and alteration of the normal rate of cell division
in the intestine of mice. One media article took an extreme view and
suggested that ultrasound safety standards should be re-written on
the basis of the report. In fact, the article referred to a paper
that had been presented at a radiology conference. Subsequently, the
paper was presented again at the annual conference of the British
Medical Ultrasound Society, in December 1999. A paper has yet to be
published in a peer-reviewed scientific journal. The capillary bleeding
was observed in the gas-filled intestines in a study using a small
number of mice. The observed effect is well known to ultrasound safety
experts, and its action depends on the presence of a tissue/gas interface.
Such a situation occurs naturally in herbivorous animals whose gut
is usually filled with gas from vegetable fermentation. While the
high attenuation coefficient of gas presents a barrier to the transmission
of ultrasound at Megahertz frequencies the impact results in damage
to tissues close to such a gas interface. The mechanism is not fully
understood but is believed to be nonthermal in nature, therefore it
is not exacerbated by the use of Doppler ultrasound. It is unlikely
to have serious consequences in human ultrasound examinations. There
is no possibility of adverse effect on the fetus from this particular
bioeffects under current diagnostic exposure conditions and in the
absence of gas bodies or echo-contrast media.
Different modes
of ultrasound application use different pulsing conditions and, therefore
may elicit different types of physical mechanisms of interaction with
biological tissue. It is the responsibility of users of diagnostic
ultrasound equipment to assess benefit and minimise risk from each
ultrasonographic examination. Responsible ultrasound societies and
organizations maintain an ultrasound safety committee or expert group
whose function is to monitor and analyse data from scientific research
and to disseminate current information in a form that is acceptable
to members. The primary purpose of this article is to draw attention
to the existence of the ASUM Safety Committee, to briefly summarise
some current issues and include publication of the current set of
ASUM safety guidelines. For more detailed information, readers are
referred to recently published comprehensive reviews and reports (Barnett
and Kossoff 1998; Barnett et.al. 2000; ter Haar and Duck 2000; WFUMB
1998). The ASUM Safety Committee comprises a small group of internationally
renowned experts in a range of disciples. The committee has enormous
experience (due to their advancing years and continued dedication
to ensuring appropriate use of ultrasound in medicine) and includes;
Dr S Barnett (CSIRO radiation biologist), Prof L de Crespigny (obstetrician),
Prof M Edwards (past-Dean Veterinary Clinical Sciences Sydney University),
Dr G Kossoff (physicist, retired).
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